Human Defensins
Among many other elements, key components of innate immunity are the antimicrobial peptides (AMPs) that individually show considerable selectivity, but collectively are able to rapidly kill a broad spectrum of bacteria, viruses and fungi. The biological significance of AMPs is emphasized by their ubiquitous distribution in nature and they are probably produced by all multicellular organisms. In humans the predominant AMPs are the defensins. The human defensins are small cationic peptides that can be divided into α- and β-defensins based on the topology of their three intramolecular cysteine disulphide bonds. The α-defensins can be further subdivided into those that were first isolated from neutrophil granules (HNP1-4) and those that are expressed by Paneth cells in the crypts of the small intestine (HD5 and HD6). The β-defensins are mainly produced by epithelial cells in various tissues and organs including the skin, trachea, gastrointestinal tract, urogenital system, kidneys, pancreas and mammary gland. The best characterized members of the β-defensin family are hBD1-3. However, using various bioinformatics tools almost 40 open reading frames encoding putative β-defensin homologues have been annotated in the human genome. Some of the human defensins are produced constitutively, whereas others are induced by proinflammatory cytokines or exogenous microbial products.
It has become increasingly clear that the human defensins in addition to their direct antimicrobial activity also have a wide range of immunomodulatory/alternative properties. These include the induction of various chemokines and cytokines, chemotactic and apoptotic activities, induction of prostaglandin, histamine and leukotriene release, inhibition of complement, stimulation of dendritic cell maturation through toll-like receptor signalling and stimulation of pathogen clearance by neutrophils. Furthermore, the human defensins also play a role in wound healing, proliferation of epithelial and fibroblast cells, angiogenesis and vasculogenesis.
There is increasing evidence that the human defensins play an important role in many infectious and inflammatory diseases. Overexpression of human defensins is often observed in inflamed and/or infected skin most likely because of local induction by microbial components or endogenous proinflammatory cytokines. In psoriasis hBD2 and hBD3 are overabundant and in lesional epithelium of patients with acne vulgaris or superficial folliculitis a significant upregulation of hBD2 has been observed. On the other hand, downregulation of hBD2 and hBD3 has been associated with atopic dermatitis. Ileal Crohn's disease has been associated with deficient expression of HD5 and HD6 and in Crohn's disease in the colon expression of hBD2-4 are downregulated.
Inflammatory Bowel Diseases
Inflammatory bowel diseases (IBD) are defined by chronic, relapsing intestinal inflammation of obscure origin. IBD refers to two distinct disorders, Crohn's disease and ulcerative colitis (UC). Both diseases appear to result from the unrestrained activation of an inflammatory response in the intestine. This inflammatory cascade is thought to be perpetuated through the actions of proinflammatory cytokines and selective activation of lymphocyte subsets. In patients with IBD, ulcers and inflammation of the inner lining of the intestines lead to symptoms of abdominal pain, diarrhea, and rectal bleeding. Ulcerative colitis occurs in the large intestine, while in Crohn's, the disease can involve the entire GI tract as well as the small and large intestines. For most patients, IBD is a chronic condition with symptoms lasting for months to years. It is most common in young adults, but can occur at any age. It is found worldwide, but is most common in industrialized countries such as the United States, England, and northern Europe. It is especially common in people of Jewish descent and has racial differences in incidence as well. Recent studies have identified variations in specific genes, including ATG16L1, IL23R, IRGM, and NOD2, that influence the risk of developing Crohn disease. As many as 30 human genes have been identified which contribute to ulcerative colitis susceptibility.
The clinical symptoms of IBD are intermittent rectal bleeding, cramping abdominal pain, weight loss and diarrhea. Diagnosis of IBD is based on the clinical symptoms, the use of a barium enema, but direct visualization (sigmoidoscopy or colonoscopy) is the most accurate test. Protracted IBD is a risk factor for colon cancer, and treatment of IBD can involve medications and surgery.
Some patients with UC only have disease in the rectum (proctitis). Others with UC have disease limited to the rectum and the adjacent left colon (proctosigmoiditis). Yet others have UC of the entire colon (universal IBD). Symptoms of UC are generally more severe with more extensive disease (larger portion of the colon involved with disease).
The prognosis for patients with disease limited to the rectum (proctitis) or UC limited to the end of the left colon (proctosigmoiditis) is better than that of full colon UC. Brief periodic treatments using oral medications or enemas may be sufficient. In those with more extensive disease, blood loss from the inflamed intestines can lead to anemia, and may require treatment with iron supplements or even blood transfusions. Rarely, the colon can acutely dilate to a large size when the inflammation becomes very severe. This condition is called toxic megacolon. Patients with toxic megacolon are extremely ill with fever, abdominal pain and distention, dehydration, and malnutrition. Unless the patient improves rapidly with medication, surgery is usually necessary to prevent colon rupture.
Crohn's disease can occur in all regions of the gastrointestinal tract. With this disease intestinal obstruction due to inflammation and fibrosis occurs in a large number of patients. Granulomas and fistula formation are frequent complications of Crohn's disease. Disease progression consequences include intravenous feeding, surgery and colostomy.
IBD may be treated medicinally. The most commonly used medications to treat IBD are anti-inflammatory drugs such as the salicylates. The salicylate preparations have been effective in treating mild to moderate disease. They can also decrease the frequency of disease flares when the medications are taken on a prolonged basis. Examples of salicylates include sulfasalazine, azulfidine, olsalazine, and mesalamine. All of these medications are given orally in high doses for maximal therapeutic benefit. These medicines are not without side effects. Azulfidine can cause upset stomach when taken in high doses, and rare cases of mild kidney inflammation have been reported with some salicylate preparations.
Corticosteroids are more potent and faster-acting than salicylates in the treatment of IBD, but potentially serious side effects limit the use of corticosteroids to patients with more severe disease. Side effects of corticosteroids usually occur with long term use. They include thinning of the bone and skin, increased susceptibility to infections, diabetes, hypertension, glaucoma, muscle wasting/weakness, rounding of the face, psychiatric disturbances, and, on rare occasions, destruction of hip joints.
In IBD patients that do not respond to salicylates or corticosteroids, medications that suppress the immune system are used. Examples of immunosuppressants include azathioprine and 6-mercaptopurine. Immunosuppressants used in this situation help to control IBD and allow gradual reduction or elimination of corticosteroids. However, immunosuppressants render the patient immuno-compromised and susceptible to many other diseases.
A well recognized model for studying IBD is the DSS colitis mouse model, as described in Kawada et al. “Insights from advances in research of chemically induced experimental models of human inflammatory bowel disease”, World J. Gastroenterol., Vol. 13 (42), pp. 5581-5593 (2007); and Wirtz and Neurath “Mouse models of inflammatory bowel disease”, Advanced Drug Delivery Reviews, Vol. 59 (11), 1073-1083 (2007).
Treatment of inflammatory bowel disease (IBD) has traditionally been accomplished with administration of aminosalicylates, corticosteroids, thiopurines, methotrexate, and anti-tumor necrosis factor agents. Human beta defensins has long be thought to play a role in the occurrence and/or treatment of IBD.
Clearly there is a great need for agents capable of preventing and treating IBD.
WO 2007/081486 discloses the use of several human defensins in the treatment of inflammatory bowel disease. The inventors suggested that defensins administered orally to Crohn's patients, in a formulation that allow their release at proper locations in the intestinal lumen, would reduce the number of invading bacteria, re-establish a normal epithelial barrier function and, thus, reduce the severity of the inflammatory disease.
According to WO 2007/081486, the function of the defensins is to directly target and kill bacteria in the lumen to prevent them from invading the epithelial tissue. That is, the function of the defensins is purely as an anti-infective compound.